Ionization detector sensitivity may be measured in a plot of detector response versus analyte concentration or analyte quantity. The range over which the detector sensitivity is constant is called the linear dynamic range, and the entire range over which response varies with concentration or quantity is called the dynamic range of the detector. The presence of a substantial concentration of analyte molecules in the detection zone of an ionization detector will consume a significant portion of the available concentration of ionizing particles. If the concentration of analyte molecules increases further, ionization of the analyte molecules can occur only at a decreasing rate and the detector response factor can be expected to decrease progressively. The upper limit of the dynamic range is determined when detector sensitivity falls to an unusable value, typically zero, and the detector is said to be saturated. The lower limit of the dynamic range occurs at a minimum detectable level (MDL).
Conventional ionization detectors suffer from nonlinearity due to the limited number of ionizing particles available for ionization, and as a result, the conventional ionization detector exhibits a linear dynamic range that is less than desirable. Particular examples of ionization detectors include the electron capture detector and the discharge ionization detector.
Electron capture detectors for gas chromatography are well known in the art. The electron capture detector (ECD) is extremely sensitive to certain molecules such as alkyl halides, but is relatively insensitive to hydrocarbons, alcohols, ketones, etc. This type of detector features high sensitivity and high selectivity towards electrophilic compounds and is widely used for detecting trace amounts of pesticides in biological systems and in food products. Such compounds typically contain halogens which combine with free electrons created in an ionization cell in the detector. The resulting decrease in free electrons in the ionization cell is monitored and used as an indication of the concentration of the compounds in a sample.
The response of the typical electron capture detector has been observed to be dependent upon many variables, such as the molecular composition of the sample and its concentration, the cleanliness and temperature of the detector cell, and the flow rates of the make-up gas and effluent. However, the behavior of the electron capture detector with regard many of these variables is not completely understood. For example, under apparently unvarying conditions, some constant current electron capture detectors can exhibit symptoms of a nonlinear and unpredictable relationship between the measured response and analyte concentration.
A discharge ionization detector operates by applying a high voltage across discharge electrodes that are located in a gas-filled chamber. In the presence of a noble gas such as helium, a characteristic discharge emission of photons occurs. The photons irradiate an ionization chamber receiving a sample gas that contains an analyte of interest. Ions are produced in the ionization chamber as a result of photon interaction with ionizable molecules in the sample gas. Helium metastables are also generated in the source chamber and are found to play a role in ionization of the analyte of interest.
FIG. 1 illustrates a detector response plot 200 recorded with use of a known helium discharge ionization detector. In the illustrated detector response plot, the analyte is carbon-12 (C.sub.12). The normalized response factor should ideally be constant irrespective of the amount of the analyte introduced into the detector. As illustrated, the normalized response factor is flat in a linear dynamic range 210 but decreases in a second, non-linear region 220 when higher amounts of analyte are introduced to the detector.
Although the design of ionization detectors continues to be an object of study in the prior art, there nonetheless exists a need for an ionization detector having a detector response that exhibits an improved dynamic range.